Why do junglers use Dorans Blade?

I'm new to jungling and was watching a few streams and saw that some junglers (Lee Sin is the one that comes to mind right now) would go boots first and then 2-3 Dorans Blade. Why do they do this? I didn't know if it was just a preference or something that I didn't understand because I'm new to jungling.

Jungle used to be my main. Basically, everything depends on what your intensions are. Doran blade as the very first jungling item is viable only with a few junglers, since with doran blade you dont have such a jungle sustain and when farming longer you get to low on your hp. Doran blade increases your early level damage output significantly, so I choose doran blade when I want to gank very early.

Example - my jungle pantheon: I run pantheon with doran blade, 9-21-0 masteries and flat ad marks and quints. On level one I take spear and clear out wovles and blue. My spear on lvl one has 105 dmg thanks to the setup. After blue I am level 2, I take the leap stun and I go for either to mid/side lane or I choose to counter-gank. I usually risk and take exhaust/smite as my summoner spells. With this setup I have scored first blood in 90% of all cases.

Taking doran blade later on in jungle means that your farm / ganks werent too successful, but again you want to increase your early game damage output.

Lee sin: I start with armor/pots item combination. I have cleared jungle, tried ganking bot lane, but I havent been successful. I tp back and probably buy boots. I clear jungle, maybe I get one successful gank. My progress seems quite slow. I would probably finish my lantern and then buy fast doran blade or two just to increase my damage/sustain/hp at the moment.

Three very important jungling things

• you have to estimate the game progress well to decide which items to buy
• doran blade gives your bruiser jungler 3 stats you actually want: lifesteal, damage, health
• if you do not start with boots (which is quite common), get boots as soon as possible, that will highly increase your jungle presence (in a sense of ganks)